This invention relates to a reset circuit for a microcomputer and more particularly to a video tape recorder having a reset circuit for its microcomputer.
There has been disclosed in Japanese Patent Publication Tokkai 60-198980 a time-lapse type video tape recorder which protects its memory device by means of a back-up power source such that the key-inputted data stored therein will not be erased by a power failure. Most video tape recorders, even if provided with a battery back-up mechanism, however, lose the stored data if the supply of power from an external source is stopped for a long time, say, for 30-60 minutes. On the other hand, video tape recorders of a so-called time-lapse type are used for monitoring, for example, and are frequently operated for a long period of time according to a stored program without any control from outside. For this reason, they are usually designed such that the stored data and programs are not lost even if the supply of power from its normal external source is discontinued for a relatively long time.
In many video tape recorders, the memory device for storing such data and programs for operation is usually provided in a microcomputer. In the event of a runaway, such a microcomputer can be reset by stopping the supply of power to it and this can be accomplished in less than an hour in the case of a video tape recorder of an ordinary type. In the case of a time-lapse video tape recorder, however, the supply of power may have to be discontinued for a much longer period, say, for about 100 hours. This is because a back-up power source for such a video tape recorder is usually capable of supplying a back-up voltage to the memory device for a longer time. For this reason, time-lapse video tape recorders are usually provided with a reset switch dedicated to the resetting of the microcomputer.
In addition to a reset switch, some time-lapse video tape recorders are also provided with a locking mechanism for "locking" a mode of operation so as to prevent, for example, an inadvertent interruption of recording caused by an operation error. Some locking mechanisms are so designed that they can be unlocked only by an authorized person. If the reset switch of a time-lapse video tape recorder is operated to reset its microcomputer, however, the mode of operation in which the apparatus has been locked becomes unlocked automatically. Since a reset switch is usually provided at an easily visible position and its method of operation can be easily learned even by an unauthorized person, the operation of a time-lapse video tape recorder in an intended mode may be frequently disrupted.